I. Field of the Invention
This invention relates generally to battery-powered, implantable medical devices, and more particularly to a method of operating such medical devices in a way that conserves battery power and thereby prolongs the operating life of the implantable device.
II. Discussion of the Prior Art
A number of different implantable medical devices are used to provide electrical stimulation to target tissue. While the present invention will be described in the context of implantable cardiac rhythm management devices, the present invention is not to be limited to that particular application, but may find use in neural stimulators, implantable drug delivery devices, etc.
Cardiac rhythm management devices include bradycardia pacemakers, anti-tachycardia pacemakers and implantable automatic defibrillators. Each of these devices includes one or more sensors for detecting cardiac depolarization signals. Included may be both atrial and ventricular sense amplifiers that are connected to provide inputs to a microprocessor-based controller. The microprocessor-based controller, operating under control of a program is designed to control the time at which a pulse generator is activated to deliver cardiac stimulating pulses to target tissue in one or both ventricles, or in the case of DDD pacemakers, to atrial tissue as well. In addition to sensors for detecting cardiac depolarization signals, in a class of stimulators referred to as “rate adaptive”, the devices may include one or more additional sensors for detecting physiologic parameters of the patient and for providing inputs to the microprocessor-based controller for adjusting the rate at which cardiac pacing pulses are delivered to meet physiologic demand.
The sensing circuits as well as the microprocessor-based controller and the pulse generator are power consuming devices and, generally speaking, receive their power from a DC battery contained within the housing of the implantable device. Great strides have been made over the past several years in battery designs to lengthen the time between an implant and when it becomes necessary to explant the device and replace it with another. In addition to improvements in battery chemistry, implantable medical devices, such as pacemakers and implantable defibrillators, have also been programmed in a way to conserve battery power. For example, programs have been written such that selected circuits are put in a “sleep mode” which is a low power consuming state, until such time as an event occurs to “awaken” the circuitry so that it can function in its intended mode.
Rate responsive pacemakers have been devised that incorporate multiple sensors for measuring physiologic demand and for developing a control signal for the microprocessor based controller that constitutes a blend of the outputs from the multiple sensors. For example, rate responsive pacemakers may incorporate an accelerometer for detecting patient movement and a minute ventilation sensor for detecting respiratory activity as an indicator of physiologic demand. When a patient having such a pacemaker implanted goes from a resting state to a minimal level of activity, the accelerometer is the primary rate controlling signal source. As activity level increases and the body requires a greater blood supply, respiratory activity becomes an important measure of physiologic demand. A discussion of rate adaptive pacemakers having multiple sensors is contained in the Stahmann et al. U.S. Pat. No. 5,376,106 and its disclosure is hereby incorporated by reference.
The Callaghan U.S. Pat. No. 4,860,751 describes an arrangement having a power consuming sensor for measuring the partial pressure of oxygen in the blood, a parameter that is known to vary with exercise. To conserve power, that sensor is disabled when a person is at rest. A passive, piezoelectric sensor is provided for turning on power to the PO2 sensor when activity produces an output above a predetermined threshold. It is to be especially noted, however, that in the Callaghan '751 patent only the PO2 sensor is operational upon being powered up. There is no blending of the multiple sensors in arriving at the control signal for varying the stimulation pulse rate of the device.
In accordance with the present invention, in order to conserve power, one or more activity sensors is/are enabled based on the activity of a single sensor when the activity measured by the first sensor falls into a predetermined range. The microprocessor is made to execute a blending algorithm, which combines the outputs of the multiple sensors in a prescribed way to produce the delta rate control signal. Further, in accordance with the present invention, those additional sensors are disabled when the activity level measured by the single sensor or a combination of all sensors falls outside of the specified range.
It is also known that most patients' natural pacemakers, i.e., the sinus node, provides an adequate heart rate for producing a cardiac output satisfactory for low levels of exertion, but not for high levels of exertion. This is known as chronotropic incompetence. In accordance with the present invention, means are provided for determining the cross-over point for chronotropic incompetence, i.e., activity level where the device could be switched from tracking an intrinsic rate to being sensor-driven. Further, when the level of activity again drops, the device can disable the activity sensor so long as the intrinsic rate is in the range from a programmed lower rate limit to the cross-over point.
When it is considered that an average pacemaker patient spends in excess of 75% of time in sedentary activities, e.g., sleeping, watching T.V., etc., the need for power consuming physiologic sensors in rate adaptive pacemakers is unnecessary at such low levels of activity. Only when the activity level is such that an increased blood supply is needed to meet physiologic demand must the pacing device boost its stimulating rate above the lower rate limit. Then, too, until the level of activity reaches a further threshold does it become necessary to provide an increased level of power dissipation in the execution of a blended algorithm where multiple sensors have their outputs combined to produce control signals to the microprocessor for effecting an appropriate rate response.
In an article entitled “Left Ventricular Pacing Alters Cardiac Function During Exercise Compared with Bi-Ventricular Pacing: A Mid-term Prospective Study By Using a Hemodynamic Sensor”, PACE, Vol. 24, April 2002, Part II, the authors concluded that for low levels of activity, bi-ventricular pacing, which is more power consuming than only left ventricular pacing, is no more beneficial to the patient than left ventricular pacing. However, they also concluded that during maximal and daily-life exercises, left ventricular pacing exhibited a higher incidence of arrhythmic events and lower performance as compared to bi-ventricular pacing and that, consequently, left ventricular pacing-induced interventricular dyssynchronization might impair left ventricular function on exercise. In accordance with a further feature of my invention, to both conserve battery power and to provide improved patient outcomes, the present invention provides a way of switching from left ventricular pacing to bi-ventricular pacing based upon the output of an activity sensor.
It is accordingly the principal object of the invention to provide increased longevity for implantable medical devices, such as pacemakers and pacemaker/defibrillators that provide rate responsive pacing based upon levels of activity determined through the use of multiple sensors by only activating the plural sensors when physiologic demand reaches a prescribed threshold.
A further object of the invention is to reduce the power consumption of activity sensors used in rate adaptive pacemakers and pacemaker/defibrillators while still providing rate response based on activity.
Yet another object of the invention is to provide an implantable medical device capable of automatically switching to a particular operating mode only when a predetermined level of activity has been reached.